Re: [PATCH -mm v4 6/9] atmel_serial: Split the interrupt handler

["Remy Bohmer" <linux@bohmer.net>]

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Hello Haavard (and Michael),

First I want to mention that I did not found the time yet to test your
latest integration of these atmel_serial patches, and I only know that
your set of the end of December works fine. I do not know the changes
you made since posting that set and your latest patch-set.

But let me clear some things up:
The original patchset I posted, existed of a set of patches suitable
for the mainline kernel, plus an additional patch for Preempt-RT only.
So, the splitup  of the interrupt handler was also needed for
Preempt-RT, but it was not the only patch that was needed on
preempt-rt.

Now looking at this problem:
> >  * Drop the lock here since it might end up calling
> >  * uart_start(), which takes the lock.
> >    spin_unlock(&port->lock);
> >  */
> >     tty_flip_buffer_push(port->info->tty);
> > /*
> >     spin_lock(&port->lock);
> >  */
> > The same code with this comments out runs

I expect the UART generating the problem is the DBGU port. The DBGU
shares its interrupt line with the timer interrupt with the IRQF_TIMER
flag set, and thus the DBGU interrupt handler is running in
IRQF_NODELAY context. Within this context it is forbidden to lock a
normal spinlock, because a normal spinlock is converted to a mutex on
Preempt-RT; a mutex can sleep which is forbidden in interrupt context.
So, to get around this problem, this lock spinlock has to be of the
raw_spinlock_t type. The raw_spinlock_t is the normal mainline-kernel
spinlock, and as such it is not converted to a mutex, and will
therefor never sleep.

Attached a patch that changes this spinlock type. I used it in my
patchset, but your updates of December last year do not need this
patch anymore, so apparantly you changed something that has a
regression on Preempt-RT...

> > Complete Preemption (Real-Time) ok but the serials is just unusable due
> > to too many overruns (just using lrz)

This problem is not there in the December-set either. It works like a charm...

I believe I have to look at the latest set of patches, and try to find
any regressions. Do you have a location somewhere where I can download
the latest versions? Or do I need to dig through LKML to find the
latest... ;-)

Kind Regards,

Remy



2008/1/30, Haavard Skinnemoen <hskinnemoen@atmel.com>:
> On Wed, 30 Jan 2008 00:12:23 +0100
> michael <trimarchi@gandalf.sssup.it> wrote:
>
> > I'm testing this patch on an at91sam9260 on 2.6.24-rt. I'm using this
> > patch with the tclib support for hrtimer and the clocksource pit_clk.
> > These are the results:
> >
> > - Voluntary Kernel Preemption the system (crash)
> > - Preemptible  Kernel (crash)
>
> Ouch. I'm assuming this is with DMA disabled?
>
> > /*
> >  * Drop the lock here since it might end up calling
> >  * uart_start(), which takes the lock.
> >    spin_unlock(&port->lock);
> >  */
> >     tty_flip_buffer_push(port->info->tty);
> > /*
> >     spin_lock(&port->lock);
> >  */
> > The same code with this comments out runs
>
> Now, _that_ is strange. I can't see anything that needs protection
> across that call; in fact, I think we can lock a lot less than what we
> currently do.
>
> > Complete Preemption (Real-Time) ok but the serials is just unusable due
> > to too many overruns (just using lrz)
>
> Is it worse than before? IIRC Remy mentioned something about
> IRQF_NODELAY being the reason for moving all this code to softirq
> context in the first place; does the interrupt handler run in hardirq
> context?
>
> > The system is stable and doesn't crash reverting this patch.
> > I don't test with the thread hardirqs active.
>
> Ok.
>
> > >> +  ret = -ENOMEM;
> > >> +  data = kmalloc(ATMEL_SERIAL_RINGSIZE, GFP_KERNEL);
> > >> +  if (!data)
> > >> +          goto err_alloc_ring;
> > >> +  port->rx_ring.buf = data;
> > >> +
> > >>    ret = uart_add_one_port(&atmel_uart, &port->uart);
> > >>
> > Is the kmalloc correct?
> > maybe:
> > data = kmalloc(ATMEL_SERIAL_RINGSIZE * sizeof(struct atmel_uart_char),
> > GFP_KERNEL);
>
> I think you're right. Can you change it and see if it helps?
>
> I guess I didn't test it thoroughly enough with DMA
> disabled...slub_debug ought to catch such things, but not until we
> receive enough data to actually overflow the buffer.
>
> > >> @@ -1033,6 +1165,9 @@ static int __devexit atmel_serial_remove(struct platform_device *pdev)
> > >>
> > >>    ret = uart_remove_one_port(&atmel_uart, port);
> > >>
> > >> +  tasklet_kill(&atmel_port->tasklet);
> > >> +  kfree(atmel_port->rx_ring.buf);
> > >> +
> > >>
> > Why the tasklet_kill is not done in atmel_shutdown?
>
> Why should it be? If it should, we must move the call to tasklet_init
> into atmel_startup too, and I don't really see the point.
>
> Haavard
>

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[-- Type: text/x-patch; name=atmel_serial_irqf_nodelay.patch, Size: 1134 bytes --]

On PREEMPT-RT we may not block on a normal spinlock in IRQ/IRQF_NODELAY-context.
This patch fixes this by making the lock a raw_spinlock_t for the
Atmel serial console.

This patch demands the following patches to be installed first:
* atmel_serial_cleanup.patch
* atmel_serial_irq_splitup.patch

Signed-off-by: Remy Bohmer <linux@bohmer.net>
---
 include/linux/serial_core.h |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6.23/include/linux/serial_core.h
===================================================================
--- linux-2.6.23.orig/include/linux/serial_core.h	2007-12-13 13:31:27.000000000 +0100
+++ linux-2.6.23/include/linux/serial_core.h	2007-12-13 16:32:22.000000000 +0100
@@ -226,7 +226,12 @@ struct uart_icount {
 typedef unsigned int __bitwise__ upf_t;
 
 struct uart_port {
-	spinlock_t		lock;			/* port lock */
+
+#ifdef CONFIG_SERIAL_ATMEL
+	raw_spinlock_t 		lock;			/* port lock */
+#else
+	spinlock_t 		lock;			/* port lock */
+#endif
 	unsigned int		iobase;			/* in/out[bwl] */
 	unsigned char __iomem	*membase;		/* read/write[bwl] */
 	unsigned int		irq;			/* irq number */